Mixed pre-cooling improves thermal strain but not running performance in female endurance athletes exercising in the heat across the menstrual cycle

This study investigated the effect of mixed pre-cooling (cooling vest and ice-slurry) on thermal and cardiovascular strain and performance in unacclimatized females exercising in the heat during menstrual cycle (MC) Phase-1 (low estrogen and progesterone) and Phase-4 (moderate estrogen and high progesterone). In a randomized crossover design, 11 runners completed two trials in each MC Phase: mixed pre-cooling (wearing a cooling vest around the torso for 30 min and ingesting 7.5 g.kg -1 of body mass of ice slurry), and control (equivalent slurry volume at ~20°C without a cooling vest), prior to a 10-km running time trial (TT) in 34°C and 60% relative humidity. Rectal temperature (T re), heart rate (HR), and perceptual responses were measured throughout the TT. Linear mixed models were fitted to estimate intervention and MC phase effects. Mixed pre-cooling had no effect on HR (1.06 [-0.83, 2.95] beats.min -1; p  = 0.272) or TT completion time (-0.23 [-1.32, 0.86] min; p  = 0.677) versus control. T re (-0.21 [-0.34, -0.07] °C; p  = 0.003) and thermal sensation (-0.61 [-0.78, -0.44] au; p  < 0.001) were significantly lower with mixed pre-cooling compared to control. Rise in T re (-0.1 [-0.3, 0.0] °C) and thermal sensation were attenuated in MC phase-4 (-0.57 [-0.81, -0.34] au; all p  < 0.05). Mixed pre-cooling was effective at attenuating the rise in T re for a similar work rate, particularly in MC phase-4, where females may experience greater heat thermosensation. However, the magnitude of attenuated T re does not improve 10-km running performance in the heat, suggesting additional cooling and/or heat mitigation strategies may be required to improve performance.